Tetrarchy

The Tetrarchy was a period of Roman history from 293 to 313 AD, during which the Roman Empire was divided into two halves and ruled by tetrarchs, with an Augustus (senior emperor) and Caesar (junior emperor) ruling over each half. The tetrarchy was created by Diocletian to end the dynastic succession system in Rome and replace it with a merit-based system; the empire would be ruled through a college of four emperors led by two senior emperors and two junior emperors. The Caesars would be chosen based on their experience and qualifications, and could not be blood relatives of the Augusti. Diocletian became Augustus of the Eastern Roman Empire with Galerius as Caesar, while the Western Roman Empire was ruled by Augustus Maximian and Constantius I as Caesar. Each emperor had principal responsibility for a distinct set of provinces, but the empire was not formally divided, with the tetrarchs serving as co-rulers of a unified empire. When an Augustus died or abdicated, his Caesar would then move up and assume his vacated position, and a new Caesar would be chosen by the most senior of the two Augusti; the new junior emperor would be a figure uniquely qualified for the position. The tetrarchy was a completely novel and ingenious conception which could continue indefinitely, but the children of current rulers could hardly be expected to accept the new system passively, and  they did not. Diocletian and Maximian abdicated in 305 to let their Caesars Galerius and Constantius rise to the senior offices, and Galerius chose Maximinus II as co-emperor, while Flavius Valerius Severus served as Constantius' Caesar. After Constantius' death in 306, Constantius returned to dynastic succession by choosing Constantine as his successor. Maximian, the retired Eastern Emperor, also had an adult son, Maxentius, who, along with Constantine, had felt slighted by being bypassed for a place in the imperial college at his father's abdication. After Constantine's army acclaimed him ruler, Maxentius urged the Praetorian Guardin Rome to proclaim him emperor, and he assumed control of Rome and Italy, ending the "Rule of Four" with a fifth member; Maxentius then brought his father Maximian out of retirement to assist him, leading to the Dissolution of the Tetrarchy.